1. Field of the Invention
The present invention relates to a valve assembly, and more particularly, to a valve assembly of a shock absorber, which can achieve both an effect of varying an attenuation force according to a frequency region of vibration or shock transferred to the shock absorber during the driving of an automobile and an effect of varying an attenuation force according to an additional pressure and can increase the attenuation force in response to an instantaneous input of a large amplitude behavior.
2. Description of the Prior Art
In general, a vehicle continuously receives vibration or a shock from a road surface through the wheels during driving. Thus, a shock absorbing device is provided between the vehicle body and the vehicle axle so as to prevent the shock or vibration from being directly transferred to the vehicle body, thereby improving ride comfort. In addition, the shock absorbing device suppresses the irregular vibration of the vehicle body, so as to enhance the driving stability.
A suspension system, which is a general name of a connection device between a vehicle axle and a vehicle body including the shock absorbing device as described above, includes a chassis spring configured to relieve shock, a shock absorber configured to control the free vibration of the chassis spring so as to improve ride comfort, a stabilizer configured to prevent rolling, a rubber bushing, and a control arm.
In the suspension system, especially, the shock absorber serves to suppress and attenuate vibration from the road surface and is mounted between the vehicle body or a frame and a wheel. In particular, the shock absorber absorbs the vibration energy of the vehicle body in the vertical direction, so as to suppress vibration, improve the ride comfort, and protect cargo on board. In addition, the shock absorber increases the service life of the vehicle by reducing dynamic stresses at each portion of the vehicle body and secures tires' road holding by suppressing the motion of a mass below the spring. Furthermore, the shock absorber improves the motional performance of the vehicle by suppressing change in the vehicle's posture caused by an inertial force.
Accordingly, the ride comfort and handling stability of the vehicle may be suitably adjusted according to the attenuation force characteristic of the shock absorber. That is, during the normal driving of the vehicle, it may be needed to reduce the attenuation force so as to improve ride comfort. Further, when quickly turning the vehicle or during the high-speed driving of the vehicle, it may be needed to increase the attenuation force so as to enhance the handling stability.
FIG. 1 illustrates a conventional shock absorber.
As illustrated in FIG. 1, the shock absorber 1 includes a cylinder 2 filled with hydraulic fluid, a piston rod 3 of which one end is positioned inside the cylinder 2 and the other end extends to the outside of the cylinder 2, and a piston valve 4 mounted at the one end of the piston rod 3 to reciprocate in the cylinder 2.
The cylinder 2 may include an inner tube 2a and an outer tube 2b, and a base valve 5 is installed at the lower end of the cylinder 2 to face the piston valve 4.
The inside of the cylinder 2 is divided into an extension chamber C1 and a compression chamber C2 by the piston valve 4. When the piston valve 4 reciprocates up and down within the cylinder 2, the hydraulic fluid flows from the extension chamber C1 to the compression chamber C2 or from the compression chamber C2 to the extension chamber C1 through an orifice (not shown) formed in the piston valve 4, thereby generating an attenuation force.
The conventional shock absorber 1 configured as described above is adapted to generate the attenuation force using a pressure difference between the tension chamber C1 and the compression chamber C2 which occurs according to the rectilinear reciprocation of the piston rod 3 connected to the vehicle body. Thus, when the moving stroke of the piston rod 3 is large or at a low frequency shock region, a proper attenuation force is generated to smoothly absorb vibration. However, when the moving stroke of the piston rod 3 is small or at a high frequency shock region, the conventional shock absorber 1 is problematic.
That is, for example, when a high frequency vibration or shock having a small amplitude and frequent vibration is applied, the pressure difference between the extension chamber C1 and the compression chamber C2 is too small to enable the piston valve 4 to smoothly operate, which results in failure in obtaining a proper attenuation force. As a result, such vibration is transferred to a rider without being completely absorbed, so as to degrade the ride comport.
Therefore, it is necessary to control the attenuation force according to the frequency difference as well as the input speed of the shock. To this end, a shock absorber having a frequency sensitive valve device additionally provided under a main valve unit vertically dividing a cylinder into an upper part and a lower part has been disclosed in Korean Utility Model Laid-Open Publication No. 20-1995-0011204 (Patent Document 1).
However, the conventional shock absorber having a frequency sensitive valve device does not have any means for controlling the pressure additionally generated according to the speed change, besides the main valve unit. Further, although it is necessary to increase the attenuation force in order to stably control the behavior of the vehicle body at a high speed operation in which the piston rod moves at a high speed due to application of an instantaneous large impact, the conventional shock absorber undergoes loss of the attenuation force because the hydraulic fluid is drained through a bypass fluid channel formed at one side of the frequency sensitive valve device in the conventional shock absorber.
For example, in the case of the shock absorber disclosed in Patent Document 1, when a low frequency shock is input, the oil flows not only through the piston valve but also through the orifice of the rotary valve. Therefore, this conventional shock absorber undergoes loss of the attenuation force through the rotary valve at the time of high speed behavior.